After impoundment of the cascade reservoirs, the incoming suspended sediment in the Three Gorges Reservoir (TGR) greatly decreased, and the asynchronous characteristics of the sediment peak and flood peak in the TGR changed simultaneously. Therefore, it is necessary to study the asynchronous characteristics of the flood peak and sediment peak and their causes, which can provide a theoretical basis for understanding the mechanism of the asynchronous characteristics of the flood peak and sediment peak and for optimizing the regulation of reservoir sediment peak discharge. The observed data of the hydrological station in the TGR from 2003 to 2018 and a theoretical analysis were used to analyse the new asynchronous characteristics of the flood peak and sediment peak in the TGR after the impoundment of the Cascade Reservoirs and to preliminarily explore the factors affecting the asynchronous changes. It was found that after the impoundment of the Cascade Reservoirs, the asynchronism of the flood peak and sediment peak in the TGR was aggravated, which revealed that the proportion of the hysteretic sediment peak increased, the propagation time of the flood peak decreased, and the propagation time of the sediment peak increased. The reasons for these changes were the source of incoming sediment being switched to the TGR, the reduction in the flood peak and the increased particle size of the suspended sediment.